Equine herpesvirus 1 (EHV-1), a member of the Alphaherpesvirinae, is the major cause of virus-induced abortion in equids and causes respiratory and neurological disease. The entire DNA sequence of the EHV-1 strain Ab4p has been determined (Telford, E. A. R. et al., 1992, Virology 189:304-316); however, only few genes and gene products have been characterized for their relevance for the virulence of EHV.
For control of EHV-1 infections, two different approaches are followed. First, modified live vaccines (MLVs) have been developed, including the strain RacH (Mayr, A. et al., 1968, J. Vet. Med. B 15:406-418; Hübert, P. H. et al., 1996, J. Vet. Med. B 43:1-14), which is widely used in Europe and the United States. Second, inactivated vaccines and independently expressed viral glycoproteins have been assessed for their immunogenic and protective potential. Among the glycoproteins that were expressed using recombinant baculoviruses are the glycoproteins (g) B, C, D, and H, which induced partial protection against subsequent challenge EHV-1 infection in a murine model (Awan, A. R. et al., 1990, J. Gen. Virol. 71:1131-1140; Tewari, D. et al., 1994, J. Gen. Virol. 75:1735-1741; Osterrieder, N. et al., 1995, Virology 208:500-510; Stokes, A. et al., 1996, Virus Res. 40:91-107). However, the use of MLVs has advantages over killed and subunit vaccines. MLVs are highly efficient in inducing cell-mediated immune responses, which are most likely to be responsible for protection against disease (Allen, G. P. et al., 1995, J. Virol. 69:606-612; Mumford, J. A. et al., 1995, Proceedings 7th International Conference of Equine Infectious Disease (H. Nakajima and W. Plowright, Eds. 261-175 R & W Publ., Newmarket, U.K. United Kingdom).
Herpesvirus glycoproteins are crucially involved in the early stages of infection, in the release of virions from cells, and in the direct cell-to-cell spread of virions by fusion of neighboring cells. To date, 11 herpes simplex virus type 1 (HSV-1)-encoded glycoproteins have been identified and have been designated gB, gC, gD, gE, gG, gH, gI, gJ, gK, gL, and gM. HSV-1 mutants lacking gC, gE, gG, gI, gJ, and gM are viable, indicating that these genes are dispensable for replication in cultured cells. Comparison of the HSV-1 and equine herpesvirus 1 nucleotide sequences revealed that all of the known HSV-1 glycoproteins are conserved in EHV-1. According to the current nomenclature, these glycoproteins are designated by the names of their HSV-1 homologs. It is known that EHV-1 gC, gE and gI are not essential for growth in cell culture, whereas gB and gD are essential for virus growth in cultured cells. The contributions of other EHV-1 glycoproteins to replication in cultured cells are not known (Flowers, C. C. et al., 1992, Virology 190:307-315). Six envelope glycoproteins of EHV-1 were mapped by using a λgtl 1 expression library and monoclonal antibodies (mAbs) raised against purified EHV-1 (Allen, G. P. et al, 1987, J. Virol. 61:2454-2461). In addition, transcriptional and protein analyses have shown that the glycoproteins gB, gC, gD, gG, gH, and gK are expressed in EHV-1-infected cells. Glycoprotein gM (encoded by gene UL10 [Baines, J. D. et al., 1991, J. Virol. 65:938-944; Baines, J. D. et al.,1993, J. Virol. 67:1441-1452]) is the most recent HSV-1 glycoprotein which has been analyzed in detail. It is the only reported nonessential glycoprotein which is conserved in all herpes viral subfamilies and has been described for human and murine cytomegalovirus and the Gammaherpesvirinae members EHV-2, herpesvirus saimiri, and Epstein-Barr virus. Like many herpesvirus glycoproteins, HSV-1 gM is present in virions and membranes of infected cells. HSV-1 mutants solely lacking gM grew to titers reduced approximately 10-fold relative to those of wild-type virus and showed a reduced virulence in a murine model (Baines, J. D. et al., 1991, J. Virol. 65:938-944; MacLean, C. A. et al., 1993, J. Gen. Virol. 74:975-983). The EHV-1 gM homolog (gp21/22a; referred to as EHV-1 gM from now on) was first described by Allen and Yeargan (Allen, G. P. et al, 1987, J. Virol. 61:2454-2461) and was shown to be a major constituent of the virus envelope. Further investigations revealed that gene 52, the gene homologous to HSV-1 UL10, encodes the 450-amino-acid EHV-1 gM polypeptide (Pilling, A. et al., 1994, J. Gen. Virol. 75:439-442; Telford, E. A. R. et al., 1992, Virology 189:304-316). EHV-1 gM represents a multiple hydrophobic protein which contains eight predicted transmembrane domains and has been reported to be present in infected cells and in purified virions as an Mr 45,000 protein (Pilling, A. et al., 1994, J. Gen. Virol. 75:439-442; Telford, E. A. R. et al., 1992, Virology 189:304-316).
In 1996 Osterrieder et al. (Virology 208:500-510) concluded from experiments that compared penetration characteristics of a viral mutant (L11ΔgM) bearing an Escherichia coli lac Z gene inserted into the EHV-1 strain RacL11 gM gene (open reading frame 52) with those characteristics of the parental EHV-1 RacL11 that the EHV-1 gM plays important roles in the penetration of virus into the target cell and in spread of the virus from cell to cell. In 1997, Neubauer et al. (Virology, 239:36-45) demonstrated that the above described EHV-1 insertion mutant of gM is attenuated and elicits protective immunity as demonstrated by the evaluation of virus-neutralizing antibodies and EHV-1-specific T-cells in spleens of immunized mice.
The technical problem underlying this invention was to provide new modified equine herpes viruses that demonstrate significantly improved immunogenic properties when used for the prophylaxis and treatment of EHV infections.